1. Field of the Invention
The present invention relates to a method of measuring shading characteristics in an apparatus for reading recorded image information from a recording medium by scanning the recording medium with a light beam to emit light representative of the image information and detecting the emitted light.
2. Description of the Prior Art
There are known image information reading apparatus in which a light beam is applied to scan a recording medium with image information recorded thereon and light reflected by, passing through, or emitted from the recording medium is detected to read the recorded image information. Heretofore, such an image information reading apparatus is employed in an image input unit for a computer, an image reader in a facsimile transmitter/receiver, or the like, for example. As one type of such an image information reading apparatus, a radiation image information reading apparatus is disclosed in Japanese Laid-Open Patent Publication No. 56-11397 in which a stimulating light beam is applied to a stimulable phosphor sheet with radiation image information of a human body, for example, recorded thereon to cause the irradiated area of the sheet to emit light representative of the radiation image information, which is then detected to produce an image signal bearing the radiation image information.
As disclosed in the aforesaid publication, the image information reading apparatus has a light detector connected to a light guide for detecting the emitted light efficiently. More specifically, the light guide that is made of a light-transmissive material has one end as a light entrance and the opposite end as a light exit to which the light detector such as a photomultiplier is connected. By arranging the light guide so that its light entrance extends along the light beam scanning line on the recording medium or stimulable phosphor sheet, light emitted from each scanned point on the recording medium enters the light guide through the light entrance, and is efficiently collected and guided to the light detector.
Where the emitted light is detected through the light guide, however, the output signal from the light detector may be varied by shading of the light guide (which is caused by an area of low light transmitting efficiency at an end of the light guide).
Japanese Laid-Open Patent Publication No. 62-16666, for example, discloses a light detector comprising an elongate photomultiplier having a sufficient long light detecting surface extending along the main scanning line on a recording medium. The disclosed elongate photomultiplier however has light sensitivity irregularities in the longitudinal direction thereof.
Moreover, the intensity of light read by the light detector may be varied in the longitudinal direction by fluctuations in the speed of angular movement of a light deflector such as a galvanometer mirror for deflecting a light beam, or irregularities in the light reflectivity or transmittance of an optical system which guides the light beam to the recording medium.
If any one of the above drawbacks occurs, the output of the light detector may be varied each time the light beams reaches a scanned position in the main scanning direction (such a phenomenon will hereinafter be referred to as "shading"). When shading takes place, it is impossible to detect light from the recording medium correctly.
For overcoming the problem of shading, it has been proposed to determine shading characteristics of the reading apparatus in the main scanning direction prior to the process of reading light emitted from the recording medium, and correct an output signal from the light detector dependent on the shading characteristics in order to cancel out shading-dependent variations in the output readout signal of the light detector.
The variations in the output readout signal from the light detector may be caused by not only the shortcomings, referred to above, in the reading apparatus, but also irregularities in the sensitivity of a stimulable phosphor sheet when radiation image information is read from the stimulable phosphor sheet or irregularities in the energy level of applied radiation when a radiation image is recorded (photographed) on a stimulable phosphor sheet. Such irregularities caused by other than the reading apparatus are frequently varied by a different stimulable phosphor sheet used or replacement of a radiation source. Therefore, if the shading characteristics are to be determined in such a manner to cover the sensitivity and radiation level irregularities as well as the various irregularities or fluctuations in the reading apparatus, there might be an instance wherein actual radiation level irregularities when recording radiation image information are directly opposite to predetermined radiation level irregularities employed when the shading characteristics have been determined. This condition would make the signal correcting process ineffective or rather make the output readout signal largely inaccurate in the signal correcting process.
In view of the aforesaid defects, it is often better to correct the output signal from the light detector based on only the shading characteristics of the image information reading apparatus.